1. Field of the Invention
This invention relates to a video signal processing apparatus capable of removing noise effectively without deteriorating the picture quality for use in a video tape recorder (VTR) or the like.
1. Description of the Prior Art
Recently, in generally used consumer VTR sets, apparatus for removing low level noise of high frequency band signal and noise removing apparatus utilizing the line correlation are used to remove noises mixed in the reproduced signals.
The noise removing apparatus for high frequency band which has been widely used is described below. FIG. 1(a) is its block diagram, in which numeral 12 is an input terminal of reproduced and demodulated luminance signal, 13 is a high pass filter, 14 is a limiter, 15 is a subtractor, and 16 is an output terminal of the signal with reduced noise.
The operation of the noise removing circuit thus composed is as follows. The high-pass filter 13 has a frequency characteristic as indicated by solid line in FIG. 1(b), for example, when the horizontal frequency f.sub.H is taken on the axis of abscissas, and the noise and signal in high band are led into the limiter 13. The limiter 13 is composed usually of semiconductor nonlinear elements, and its input-output characteristic is as shown in FIG. 2. Accordingly, when the input component of the limiter is a noise with a small amplitude, it passes through the limiter and is subtracted by the subtractor 15, and the frequency characteristic of the apparatus from the input terminal 12 to output terminal 14 becomes a low-pass performance as shown by broken line in FIG. 1(b), so that the noise in high band is removed. However, when a high band signal with a large amplitude is contained in the input signal, since the amplitude is limited by the limiter, this signal is transmitted to the output terminal 16, being hardly deteriorated from the input terminal 12.
In a consumer VTR, meanwhile, since the S/N ratio is still insufficient by the noise removing apparatus for high band as shown in FIG. 1 alone, a noise removing apparatus utilizing the line correlation is used at the same time. Referring further to the drawing, the conventional noise removing apparatus utilizing the line correlation is described. FIG. 3(a) is a schematic structural drawing of the conventional noise removing apparatus utilizing the line correlation, in which numeral 1 is an input terminal of reproduced FM luminance signal, 4 is a glass delay line for delaying the FM signal by 1H, 5 and 6 are demodulators for frequency demodulation, 7 is a subtractor for obtaining the difference between the demodulated luminance signal and the 1H delayed luminance signal, 8 is an attenuator for attenuating the output signal of the subtractor 7 to about 1/2, 9 is a limiter for limiting the amplitude of signal, 10 is an adder for adding the output signal of demodulator 5 and output signal of limiter 9, and 11 is an output terminal of signal. An example of such noise removing apparatus is disclosed in the Japanese Unexamined Patent Publication Sho. No. 55-80966.
The operation of thus composed noise removing apparatus utilizing the line correlation is as follows. In FIG. 3, the line from the input terminal to limiter 9 has the so-called C-shaped comb line filter characteristic with respect to the demodulated signals, and when the vertical frequency f.sub.V is taken on the axis of abscissas, in the case of NTSC system, the frequency characteristic becomes as shown by solid line in FIG. 3(b) (where cy/ph refers to cycles/picture height), and the noise without line correlation and signal are fed into the limiter 9. Here, the input-output characteristic of the limiter 9 is as shown in FIG. 2. Therefore, when the component without line correlation which is led into the limiter 9 has only a small amplitude to be-regarded as noise, it directly passes through the limiter. As a result, the apparatus from the input terminal 1 to the output terminal 11 has the so-called Y-shaped comb line filter characteristic with respect to the demodulated signal, so that the noise in the vicinity of the vertical frequency 525/4 (cy/ph) as indicated by broken line in FIG. 3(b) is removed. However, if the component fed into the limiter 9 is a signal without line correlation having a large amplitude, since the amplitude is limited by the limiter, the output signal from the demodulator 5 is delivered almost directly to the output terminal 11, without being removed. Thus the vertical resolution of the picture is not deteriorated so much.
Nevertheless, since thus composed conventional noise removing apparatus for high band and noise removing apparatus utilizing the line correlation are composed of analog circuits, many parts are required to make up circuits, and the structure of the VTR is much complicated. Besides, since the operation of the limiter depends on the nonlinear characteristic of semiconductor, it is difficult to realize a desired characteristic, and if the input signal amplitude is large, the output amplitude of the limiter does not become 0, so that the resolution of the picture may be deteriorated more or less. Still, in the noise removing apparatus utilizing the line correlation, since the 1H delay-is effected in the form of FM signal by using glass delay line, two demodulators are required, and yet the noise removing effect may not be exhibited sufficiently due to the instability of frequency characteristic of the glass delay line or inaccuracy of delay time. It was also difficult to reduce the size of the apparatus.